Irrigação suplementar com águas salobras como estratégia para incrementar a produtividade do milho no semiárido brasileiro

Abstract

Supplemental irrigation during dry spells could reduce maize yield losses in tropical semi arid regions, notably o n small farms. However, good quality water reserves are not sufficient to meet the demands of farming communities. So, our objective was to evaluate the productivity and profitability of maize , as well as physiological and spectral responses of mature leaves, under different simulated water scenarios, with and without supplemental irrigation with brackish water. The field experiments w ere carried out during the cropping seasons of 2018 and 2019 on Ultisol. The experiment followed a randomized block design in the split plot scheme with four replicates. The plots were formed by simulating the water supply in the soil corresponding to four water scenarios (rainy, normal, drought and severe drought), and the sub plots with or without supplemental irrigation with brackish water (e lectrical conductivity of 4. 5 dS m 1 ). The dry spell reduce the photosynthetic capacity of maize leaves, even in the normal water scenario, but the effects become drastic in the years of drought and severe drought due to stomatal and non stomatal limitations. Supplementary irrigation with brackish water reduces the water stress of maize during dry spells and improves leaf gas exchange, with a direct reflection on the CO 2 assimilation rate and on shoot biomass production. Mean yields of green maize ears were 13,083, 11,532, 10,358 and 9,609 kg ha 1 , respectively , for rainy, normal, drought and severe drought scenarios, as supplemented with brackish water. For the same treatments without supplemental irrigation the values were 11,394, 7,896, 3,913 and 1,374 kg ha 1 , respectively. Data of farmer’s income showed that under normal, drought and severe drought, supplementation with brackish water becomes economically advantageous from 0.9, 1.0 and 2.4 ha, respectively, while maize cultivation without supplemental irrigation is not advantageous for drought and severe drought scenarios . For severe drought conditions, however, supplemental irrigation using brackish water reduced commercial yield by 27%, with negative effects on the farmer’s profitability. For this case, it is necessary to use wate r of either lower salini ty or s elect crop of higher salt tolerance Supplementa l irrigation with brackish water reduces the water stress of corn during summer and improves leaf gas exchange, with a direct reflection on the rate of CO 2 assimilation and on the biomass production of the aerial part. Spectral data showed that water stress caused by dry spells is more pronounced than the salt stress associated with supplementary irrigation with brackish water The proline concentrations were weakly correlated to the studied wavelengths, being the highest in the visible range.